Horizontal diffusion furnaces have been widely used for many years for a variety of semiconductor fabrication processes including annealing, diffusion, oxidation, and chemical vapor deposition. As a result, these processes are well understood, especially with regard to the impact of process variables on the quality and uniformity of resulting products.
As semiconductor fabrication continues to advance, requiring finer dimensional tolerances and control, the semiconductor industry has become concerned that conventional horizontal tube furnace designs may not be able to meet future product requirements, particularly in connection with large wafer sizes. The semiconductor industry is particularly concerned with particulate control, automation, small critical dimensions, and shallow junctions.
Modern integrated circuit design dictates that line width be smaller than 1 micron, and that junction depth be less than a few hundred angstroms. Thus, heating duration on the wafer must be reduced to limit the lateral diffusion of the dopants, and the associated broadening of line dimension. Heating duration must also be limited to reduce forward diffusion so the junction depth does not shift. Less heating also results in less damage to wafers. Issues such as larger wafer size, wafer loading/unloading automation, and particulate control are now so critical that conventional tube furnaces can no longer meet process requirements. Two different approaches, vertical tube furnaces and single wafer rapid thermal processors, have been developed to meet the more demanding process requirements of advancing semiconductor technology. Both approaches are designed to accommodate larger wafer sizes, to be easily automated, to have a smaller footprint, and to be bulkhead mounted. Unfortunately, neither of these technologies has fulfilled its promise in terms of process performance.
The vertical diffusion furnaces are designed to support the processing tube in the vertical position. The wafers can be loaded from either the top or bottom and can be held either horizontally or vertically. This design reduces wafer contamination problems and some automation concerns. However, vertical furnaces have inherited most of the major weaknesses of the horizontal systems, such as poor process control for larger wafer sizes, excessive time-at-temperature, inconsistent wafer to wafer uniformity, and difficult process control due to the batch processing of wafers.
The Rapid Thermal Processor (RTP), in contrast, employs a high intensity light source with appropriate reflectors to heat single wafers. In a conventional RTP system, wafers are heated to temperatures of approximately 450 to 1400 degrees Centigrade and cooled to room temperature in a matter of seconds, rather than minutes as in a tube furnace. The object of Rapid Thermal Processing is to provide an alternative thermal technique that would avoid the problem of dopant profile shifting.
Currently, a number of commercial rapid thermal processors are available. Each uses some type of lamp as the heat source. A lamp source has the advantage of very low thermal mass, so that it can be powered up and down very rapidly. As a result, lamp sources can heat up the wafer very rapidly, and allow the wafer to cool more quickly. However, there are several problems associated with lamp-based heating systems which inhibit the incorporation of RTP's into production environments.
All lamp powered systems require a reflector to distribute radiated power evenly onto the wafer. Because all lamp systems use line sources of light, it is difficult to design a reflector which can evenly distribute the radiation onto wafers of different sizes. Additionally, during operation, lamps are turned on and off very rapidly. There exists no means for maintaining uniform power distribution and temperature uniformity during transient periods.
Moreover, all lamps age with time, so that it is difficult to maintain repeatable performance. The heat source of lamp powered systems is separated from the processing chamber by a quartz plate, and any contamination on this plate will affect temperature uniformity.
In addition to these heat source limitations, RTPs have still other deficiencies. For example, the processing chamber is characterized by design requirements which conflict during the heating and cooling periods. In particular, when temperature is ramping up, the processing chamber should ideally have minimum absorption, to reduce the power requirement. When temperature is ramping down, the processing chamber should have maximum absorption, to speed wafer temperature drop. As a result of these two conflicting design requirements, chamber design cannot be optimized.
Further, the switching of large lamp current during operation can generate correspondingly large magnitudes of electro-magnetic interference (EMI), which affect nearby electronic devices. Lastly, lamp-based heating systems do not use power efficiently. Consequently, large power and facility resources are required.
RTPs have not proven to be a reliable means of obtaining consistently uniform and repeatable results. One of the persistent problems associated with Rapid Thermal Processing is the difficulty in maintaining a uniform temperature across a wafer that is essentially floating in free space while being almost instantaneously heated, processes and cooled. No rapid thermal processor manufacturer has been able to solve the complex optical design requirements necessary to achieve consistently satisfactory uniformity across a wafer. Additionally, all rapid thermal processors suffer the effects of light flux degradation caused by aging of the light source, resulting in progressive degradation of repeatability.
Accordingly, there exists a need for a semiconductor fabrication and processing system which can accommodate large semiconductor wafers while maintaining satisfactory and repeatable process control, especially temperature uniformity and stability.
It is thus an object of the invention to provide an improved semiconductor fabrication and processing furnace.
It is another object of the invention to provide an improved semiconductor processing furnace which maintains enhanced temperature uniformity across the semiconductor wafer during processing.
It is a further object of the invention to provide a semiconductor processing furnace which provides rapid cooling of the wafer, and which maintains a rapid heating rate.
It is still another object of the invention to provide a semiconductor fabrication furnace which is characterized by low cost, power consumption, and maintenance requirements.
Other general and specific objects of the invention will in part be obvious and will in part appear hereinafter.